For reasons of safety, the tire pressure of tires of a motor vehicle must be regularly checked which is often not done by the driver of a motor vehicle for various reasons. Accordingly, tire pressure control systems have been developed which include a tire pressure control device allocated to each wheel. The tire pressure control device automatically measures the tire pressure of the vehicle tire and announces at least a critical deviation from the desired tire pressure to the driver of the vehicle. The tire pressure control devices can, for example, be vulcanized into the tires or attached with adhesive or attached to the valve or at the wheel rim. Appropriate configurations are known.
German patent publication 4,205,911 discloses a tire pressure control system wherein each tire of the motor vehicle is assigned a corresponding tire pressure control device. At regular intervals, each tire pressure control device transmits a measured pressure signal together with an individual identifier to a central unit. The transmission of the individual identifier prevents the situation that the data, which is transmitted to the central unit, can be confused, for example, with data which is transmitted from another motor vehicle. In the central unit, value pairs (identifier of tire pressure control device/wheel position) are stored for each wheel so that a conclusion can be drawn in the central unit via a corresponding comparison as to which identifier is transmitted with the corresponding pressure signal from which wheel position of the vehicle. A deviation of the transmitted pressure signal from a pregiven value at a wheel position is indicated to the driver of the motor vehicle by the central unit so that the driver can initiate suitable measures.
German patent publication 4,205,911 discloses that the tire pressure control system can only function without difficulty when the allocations (identifier of the tire pressure control devices/wheel position) are properly stored in the central unit. Correspondingly and at least after each change of tires on the vehicle, a new allocation must be undertaken which takes place in an allocation mode of the tire pressure control system. In the tire pressure control system disclosed in German patent publication 4,205,911, a new allocation is carried out in that the intensity of the signals, which are transmitted from the individual tire pressure control devices, is measured by receivers (of which one each is fixedly assigned to a wheel position) and each signal (which is transmitted from a tire pressure control device) is assigned to the wheel position of the vehicle at which it generates the highest signal intensity. For example, the signal intensity of the tire pressure control device, which is located in the forward left tire, is the largest at the receiver, which is allocated to the forward left wheel position, so that a corresponding allocation can be determined. The corresponding allocations are stored in the central unit.
For the allocation method explained, a receiver is needed each wheel position of the motor vehicle whereby the costs of the tire pressure control system are driven upwardly for the system disclosed in German patent publication 4,205,911. A tire pressure control system is disclosed in U.S. patent application Ser. No. 08/854,269, filed May 9, 1997, and incorporated herein by reference. In this system, a new allocation of the tire pressure control devices to the wheel positions is carried out with the aid of only one receiver in the motor vehicle so that a cost advantage results compared to the tire pressure control system disclosed in German patent publication 4,205,911.
The basic idea in the above-identified U.S. patent application Ser. No. 08/854,269 is that a measured quantity is measured at each wheel position of the motor vehicle by two independent measuring devices. The wheel position is known from a first measuring device and the second measuring device is a component of the tire pressure control device which is located in the tire of the wheel of the motor vehicle. The first measuring device can, for example, be an ABS sensor and the second measuring device can be a rotation sensor which is assigned to the particular tire pressure control device. With the sensors, the rpms of the wheels of the vehicle are measured in the allocation mode independently of each other and the identifier of a tire pressure control device is allocated to that wheel position at which the same rpm was measured by the ABS sensor and a rotation sensor.
The tire pressure control devices to which the rotation sensors are allocated are only transmitters which themselves can receive no data. For this reason, the data cannot be transmitted to the tire pressure control devices that a new allocation must be carried out in the tire pressure control system because of a change of tires. The tire pressure control devices in the wheels of the vehicle must therefore assume that the tire pressure control system is continuously in the allocation mode. For this reason, the number of rotations of the wheels is measured continuously by the rotation sensors and the picked-up signal is continuously amplified so that an adequate signal strength is present. This loads the batteries of the tire pressure control devices whereby their service life is reduced. A desired battery service life of at least five years is difficult to achieve because of this loading.